High pressure effect on structural and mechanical properties of some LnO (Ln=Sm, Eu, Yb) compounds

The structural and mechanical properties of LnO (Ln=Sm, Eu, Yb) compounds have been investigated using a modified interionic potential theory, which includes the effect of Coulomb screening. We predicted a structural phase transition from NaCl (B₁)- to CsCl (B₂)-type structure and elastic properties in LnO compounds at very high pressure. The anomalous properties of these compounds have been correlated in terms of the hybridisation of f-electrons of the rare earth ion with conduction band and strong mixing of f-states of lanthanides with the p-orbital of neighbouring chalcogen ion. For EuO, the calculated transition pressure, bulk modulus and lattice parameter are close to the experimental data. The nature of bonds between the ions is predicted by simulating the ion-ion (Ln-Ln and Ln-O) distances at high pressure. The second order elastic constants along with shear modulus and Young's modulus, elastic anisotropy and Poisson's ratio are also presented for these oxides.     

Structural phase transition and elastic properties of Curium and Uranium monobismuthides under pressure effect

A density functional (DFT) calculations of the structural, elastic and high pressure properties of the cubic XBi (X=U,Cm) compounds, has been reported using the full potential linear muffin-tin orbital (FP-LMTO) method. In this approach the local density approximation (LDA) is used for the exchange-correlation (XC) potential. Results are given for lattice constant, bulk modulus and its pressure derivatives. The pressure transitions at which these compounds undergo structural phase transition from NaCl-type (B1) to CsCl-type (B2) phase were found to be in good agreement with the available theoretical results. We have determined the elastic constants C₁₁, C₁₂, C₄₄ and their pressure dependence which have not been established experimentally or theoretically.     

Characterisation of the high-pressure structural transition and elastic properties in boron arsenic

This paper carries out the First principles calculation of the crystal structures (zinc blende (B3) and rocksalt (B1)) and phase transition of boron arsenic (BAs) based on the density-functional theory. Using the relation between enthalpy and pressure, it finds that the transition phase from the B3 structural to the B1 structural occurs at the pressure of 113.42GPa. Then the elastic constants C₁₁, C₁₂, C₄₄, bulk modulus, shear modulus, Young modulus, anisotropy factor, Kleinman parameter and Poisson ratio are discussed in detail for two polymorphs of BAs. The results of the structural parameters and elastic properties in B3 structure are in good agreement with the available theoretical and experimental values.     

Elastic properties of alkaline earth oxides under high pressure

In this article, we have investigated the high-pressure structural phase transition of alkaline earth oxides using the three-body potential (TBP) model. Phase transition pressures are associated with elastic constants. An effective inter-ionic interaction potential (TBP) with long-range Coulomb interactions and the Hafemeister–Flygare type short-range overlap repulsion and the vdWl interaction is developed. The present calculations have revealed reasonably good agreement with the available experimental data on structural transition (B1–B2 structure). The phase transition pressures Pt of MgO, CaO, SrO, and BaO occur at 220, 45, 40, and 100?GPa, respectively. Further, the variations of the second-order elastic constants with pressure have followed a systematic trend, which are almost identical to those exhibited by the observed data measured for other semiconducting compounds with rocksalt (B1)-type crystal structure. It is found that TBP promises that we would be able to predict phase transition pressure and elastic constants for other chalcogenides as well. The results may be useful for geophysical study.     

Structural phase transition and elastic properties of cerium chalcogenides at high pressure

The structural and elastic properties of cerium chalcogenides (CeZ, Z = S, Se, Te) under high pressure have been investigated by using the potential model considered up to third nearest neighbor interaction. The computed values of B1-B2 phase transition pressure, equation of state (compression curve), bulk modulus, its first order pressure derivative and elastic constants in the case of cerium chalcogenides agree well with the experimental results. The present study shows the anomalous behavior of cerium chalcogenides in comparison to the alkaline earth chalcogenides, due to the presence of Kondo effect and reentrant valence behavior of Ce in cerium chalcogenides.     

Electronic,elastic and dynamical properties of MgSe under pressure: rocksalt and iron silicide phase

The electronic, elastic and dynamical properties of MgSe in the rocksalt (B1) and iron silicide (B28) phase and the effects of pressure on these properties are investigated using first-principles method. The calculated electronic band structure indicates that the B1 phase of MgSe presents an indirect band-gap feature and the band gaps initially increase with pressure and subsequently decrease upon compression. Remarkably, an indirect-to-direct band-gap transition has been observed at the phase transition pressure. The elastic constants, bulk modulus, shear modulus, Young’s modulus, elastic anisotropy and B/G ratio of MgSe in the B1 and B28 phase at high pressure have also been investigated. The bulk modulus, shear modulus and Young’s modulus all increase monotonously with the increasing of pressure for the B1 and B28 phase of MgSe. The calculated phonon frequencies of the B1 phase at zero pressure agree well with available theoretical results. And the transverse acoustic phonon TA(X) mode of this phase completely softening to zero at 82 GPa. The phonon curves of the B28 phase under pressure have also been successfully investigated.     

Phase transition in CeSe, EuSe and LaSe under high pressure

The high pressure phase transition and elastic behavior of rare earth monoselenides (CeSe, EuSe and LaSe) which crystallize in a NaCl-structure have been investigated using the three body interaction potential (TBIP) approach. These interactions arise due to the electronshell deformation of the overlapping ions in crystals. The TBP model consists of a long range Coulomb, three body interactions and the short range overlap repulsive forces operative up to the second neighboring ions. The authors of this paper estimated the values of the phase transition pressure and the associated volume collapse to be closer than other calculations. Thus, the TBIP approach also promises to predict the phase transition pressure and pressure variations of elastic constants of lanthanide compounds.      

Phase transition,structural, mechanical and thermal properties of erbium pnictides under pressure

In this article, we have investigated the high-pressure structural phase transition of erbium pnictides (ErX; X?=?N, P and As). An extended interaction potential model has been developed (including the zero-point energy effect in three-body interaction potential model). Phase transition pressures are associated with a sudden collapse in volume. The phase transition pressures and associated volume collapses have been predicted successfully. The elastic constants, their combinations and pressure derivatives are also reported. The pressure behaviour of elastic constants, bulk modulus and shear modulus have been presented and discussed. Moreover, the thermophysical properties such as molecular force constant (f), infrared absorption frequency (υ ₀), Debye temperature (θ _D) and Grunneisen parameter (γ) have also been predicted.     

Phase transition and high pressure behavior of Zirconium and Niobium carbides

We have predicted the phase transition pressure (P _T )and high pressure behavior of Zirconium and Niobium carbide (ZrC, NbC). The high pressure structural phase transitions in ZrC and NbC has been studied by using a two body inter-ionic potential model, which includes the Coulomb screening effect, due to the semi-metallic nature of these compounds. These transition metal carbides have been found to undergo NaCl (B1) to CsCl (B2)-type structural phase transition, at high pressure like other binary systems. We predict such structural transformation in ZrC and NbC at a pressure of 98GPa and 85GPa respectively. We have also predicted second order elastic constant and bulk modulus. The present theoretical work has been compared with the corresponding experimental data and prediction of LAPW and GGA and LDA theories.      

High pressure structural phase transition in uranium monochalcogenides

The pressure induced phase transition in uranium monochalcogenides, UX (X = S, Se, and Te) is studied by two-body potential approach. It is found that US, USe and UTe undergo a structural phase transition from NaCl (B1) type to CsCl (B2) type at 78.5, 21 and 9.5 GPa, respectively, which is in good agreement with the recent experimental data. In addition, second-order elastic constants (SOECs) (C ₁₁, C ₁₂ and C ₁₄) have been calculated which can be used to establish the nature of the forces in these materials. The present study shows that the considered two-body potential model can be used to predict the phase transition pressure in UX compounds provided the strength and hardness parameters in B1 and B2 phases are different.     

A first-principle study of the structural, elastic, lattice dynamical and thermodynamic properties of PrX (X=P, As)

The structural, phase transition, elastic, lattice dynamic and thermodynamic properties of rare-earth compounds PrP and PrAs with NaCl (B1), CsCl (B2), ZB (B3), WC (B_h) and CuAu (L1₀) structures are investigated using the first principles calculations within the generalized gradient approximation (GGA). For the total-energy calculation, we have used the projected augmented plane-wave (PAW) implementation of the Vienna Ab-initio Simulation Package (VASP). Specifically, some basic physical parameters, e.g. lattice constants, bulk modulus, elastic constants, shear modulus, Young's modulus and Poison's ratio, are predicted. The obtained equilibrium structure parameters are in excellent agreement with the experimental and theoretical data. The temperature and pressure variations of the volume, bulk modulus, thermal expansion coefficient, heat capacity and Debye temperature are calculated in wide pressure and temperature ranges. The phonon dispersion curves and corresponding one-phonon density of states (DOS) for both compounds are also computed in the NaCl (B1) structure.     

Pressure dependences of elastic and lattice dynamic properties of AlAs from ab initio calculations

Ab initio calculations, based on norm-conserving nonlocal pseudopotentials and density functional theory (DFT), are performed to investigate the structural, elastic, dielectric, and vibrational properties of aluminum arsenide AlAs with zinc-blende (B3) structure and nickel arsenide (B8₁) structure under hydrostatic pressure. Firstly, the path for the phase transition from B3 to B8₁ is confirmed by analyzing the energies of different structures, which is in good agreement with previous theoretical results. Secondly, we find that the elastic constants, bulk modulus, static dielectric constants, and the optical phonon frequencies are varying in a nearly linear manner under hydrostatic pressure. What is more, the softening mode of transversal acoustic mode at X point supports the phase transition in AlAs.     

First-principles lattice dynamical study of ScAs and ScSb at zero and high pressure

A first-principles pseudopotential method is used to investigate the structural and elastic properties of ScAs and ScSb in their ambient B1(NaCl) and in high pressure B2 (CsCl) phases and phonon structures at zero and close to phase transition pressure. The calculated lattice constants, static bulk modulus, first order pressure derivative of the bulk modulus and the elastic constants are reported in B1 and B2 structures and compared with available experimental and other theoretical results. The phonon properties of these two compounds are compared among themselves which reveal that these compounds are predominantly metallic, due to degeneracy of optical frequencies at the zone centre. At high pressure, near the B1 to B2 transition, the LA mode at X-point softens leading to structural instability.     

A polymerization-depolymerization model for generation of contractile force during bacterial cell division

We have investigated the pressure-induced phase transition of NiO and other structural properties using three-body potential approach. NiO undergoes phase transition from B1 (rocksalt) to B2 (CsCl) structure associated with a sudden collapse in volume showing first-order phase transition. A theoretical study of high pressure phase transition and elastic behaviour in transition metal compounds using a three-body potential caused by the electron shell deformation of the overlapping ion was carried out. The phase transition pressure and other properties predicted by our model is closer to the phase transition pressure predicted by Eto et al.      

Pressure induced phase transition in rare earth mono-antimonides

Pressure induced structural phase transition of mono-antimonides of lanthanum, cerium, praseodymium and neodymium (LnSb, Ln=La, Ce, Pr and Nd) has been studied theoretically using an inter-ionic potential with modified ionic charge which parametrically includes the effect of Coulomb screening by the delocalized f electrons of rare earth (RE) ion. The anomalous structural properties of these compounds have been interpreted in terms of the hybridization of f electrons with the conduction band and strong mixing of f states of Ln ion with the p orbital of neighbouring antimonide ion. All the four compounds are found to undergo from their initial NaCl (B₁) phase to body centered tetragonal (BCT) phase at high pressure and agree well with the experimental results. The body centered tetragonal phase is viewed as distorted CsCl structure and is highly anisotropic with c/a=0.82. The transition pressure of LnSb compounds is observed to increase with decreasing lattice constant in NaCl phase. The nature of bonds between the ions is predicted by simulating the ion-ion (Ln-Ln and Ln-Sb) distances at high pressure. The calculated values of elastic constants are also reported.     

High pressure behavior and structural properties of transition metal carbides

A pressure induced structural phase transition from NaCl-type (B₁) to CsCl-type (B₂) structure has been predicted in transition metal carbides, namely TiC, ZrC, NbC, HfC, and TaC by using an interionic potential theory with modified ionic charge (Z_m ), which includes Coulomb screening effect due to d-electron. The phase transition pressure (P_T ) relies on large volume discontinuity in pressure–volume relationship, and identifies the structural phase transition from B₁ phase to B₂ phase. The variation of second-order elastic constants with pressure follows a systematic trend identical to that observed in other compounds of NaCl-type structure. The Born criterion for stability is found to be valid in transition metal carbides.     

Structural,mechanical and thermal properties of some Holmium Pnictides under pressure: A theoretical approach

The high pressure structural, elastic and thermal properties of holmium pnictides HoX (X=N, P, As and Bi) were investigated theoretically by using an inter-ionic potential theory with modified ionic charge parameter. We have predicted a structural phase transition from NaCl (B₁) to CsCl (B₂)-type structure at pressure of 139 GPa for HoN, 52 GPa for HoP, 44 GPa for HoAs and 26 GPa for HoBi. Other properties, such as lattice constant, bulk modulus, cohesive energy, second and third-order elastic constants were calculated and compared with the available experimental and theoretical data. In order to gain further information the brittle behaviour of these compounds was observed. Some other properties like Shear modulus (G), Young's modulus (E), Poisson's ratio (ν), anisotropy factor (A), sound velocities, Debye temperature (θ_D) were calculated. The variation of elastic constants (C₁₁ and C₄₄) and Debye temperature (θ_D) with pressure was also presented.     

Phase transition, elastic, thermodynamic properties of zinc-blend BeSe from first-principles

The structural phase transition, elastic, thermodynamics properties of BeSe in zinc-blende were investigated by performing first-principles calculations within generalized gradient approximation. The phase transition pressure P_t between the B3 phase and the B8 phase of BeSe was determined. The pressure dependencies of elastic constants, shear modulus, Young's modulus, and Poisson's ratio of BeSe are calculated. The thermodynamic properties of the zinc-blende structure BeSe are calculated by using the quasi-harmonic Debye model. The pressure and temperature dependencies of the heat capacity and the thermal expansion coefficient, as well as the Grüneisen parameter are investigated systematically in the ranges of 0–50 GPa and 0–1200 K.     

Pressure dependent mechanical and thermodynamical properties of Hg<Subscript>0.91</Subscript>Mn<Subscript>0.09</Subscript>Te semiconductor

The mechanical, thermodynamical and elastic properties of Hg_0.91Mn_0.09Te compound are calculated by formulating an effective interionic interaction potential. This potential consists of the long-range Coulomb, three body force parameter, the Hafemeister and Flygare type short-range overlap repulsion extended upto the second neighbor ions and the van der Waals (vdW) interaction. The estimated values of phase transition pressure have revealed reasonably good agreement with the available experimental data on the phase transition pressure P _t = 11.5 GPa and the vast volume discontinuity in pressure-volume (PV) phase diagram indicate the structural phase transition from zincblende (B3) to rock salt (B1) structure. Later on, the Poisson’s ratio ν, the ratio R _S/B of S (Voigt averaged shear modulus) over B (bulk modulus), elastic anisotropy parameter, elastic wave velocity, average wave velocity and Debye temperature as functions of pressure is calculated. From Poisson’s ratio and the ratio R _S/B it is inferred that Hg_0.91Mn_0.09Te is brittle in nature in both B3 phase and B1 phase. To our knowledge this is the first quantitative theoretical prediction of the pressure dependence of ductile (brittle) nature of Hg_0.91Mn_0.09Te compounds and still awaits experimental confirmations.